Figure 6: a) Exploration of the submerged level of
water. b) Effect of heating the WS-WEG device. c) Evaluation of
different heights of the WS. d) Voltage vs time with changing polarity.
e) Effect of relative humidity level on Voc and
Jsc. f) Effect of NaCl concentration on
Voc and Jsc.
The temperature of the water reservoir was increased from 25 ºC to 85 ºC
gradually to study the effects of the temperature, which is depicted inFigure 6 (b). The results show that the Vocgradually increases up to the voltage of 720 mV after a certain time
while the temperature reaches 85 ºC. This can be attributed to the
combined effects of the enhanced movement of water molecules, elevated
evaporation rate, and thermoelectric effects.[50]In addition, five different heights of the shell structure were examined
to see the thickness effects on the Voc. Since the
thickness of the WS was under 2 mm, the cross-sectional configurations
were considered for the elevated heights experiments. The density along
the cross-section is comparatively higher, confirmed by SEM (Figure-S
3); therefore, the slightly lower voltage was observed. The results inFigure 6 (c) confirm that altered heights have a substantial
effect on the Voc. Elevated heights enhance the
variations of ion concentrations; however, the more restrictions on
water flow along the elevated cross sections reduce the output voltage.
By swapping the positive and negative electrodes on the multimeter, the
polarity and magnitude of the open circuit voltage were compared. The
results of Figure 6 (d) reveal that changing the polarity has
no major impact on the voltage; the polarity changes only caused
by the flipping. The effect of the humidity on the device performance
was evaluated systematically. The results shown in Figure 6 (e) indicate the lower performance in extremely humid environments. This is
due to the reduced evaporation rate in humid environments. While
evaluating the effects of different concentrations of NaCl, the open
circuit voltage seems to be higher at higher concentrations of NaCl,
which is represented in Figure 6 (f) . Compared to water, NaCl
leads to an enhanced voltage and current density of 703 mV and 0.78
µW/mm2 respectively. Theoretically, enhanced
concentrations of ions lead to a decrease in streaming potentials
because of the reduced size of the double
layer.[19] In this case, however, enhanced open
circuit voltage are caused by the continuous evaporations of water. The
higher solubility of NaCl and elevated migration rate of
Na+ ions associated with WS micro/nanochannels
facilitate the higher concentration gradients in the ionic
pathway.[51] The output voltage is, consequently,
therefore, boosted by the enhanced level of ions in the solutions.
Performance enhancement and power
density analyses
The acid-treated samples (WS-H+) were immersed in the
reservoir of neutral water and alkaline solutions separately as
illustrated in Figure 7 (a). WS-H+experiences a partial reduction of lignin and exposure of cellulose as
observed in CLSM (Figure 3 (f)) and FT-IR spectra
(Figure 7 (b) ). As a result of this treatment additional
nanochannels are created, as confirmed by the SEM (Figure 3
(h) ), which can be referred to as nano-engineered
WS-H+. C-O-C asymmetric stretching at 1160
cm-1, C-H bending at 1360 cm-1 and
C-O-C stretching at 1105 cm-1 confirms the presence of
cellulose and C=O at 1040 cm-1 confirms the stretching
vibration of cellulose and hemicellulose[52] which
are prominent in the nano-engineered WS-H+ samples as
confirmed by Figure 7 (b).
Upon acid treatments the WS-H+ samples were protonated
and the signals at 1738 cm-1 disappeared after being
placed on alkaline reservoir for 8 h as shown in Figure-S 9 .
The thermal gravimetric analyses at Figure 7 (c) demonstrates
that the thermal stability of the natural walnut shell (NWS) is more
than the treated WS and WS-H+. This is due to the
reduction of lignin contents after the chemical treatments since lignin
has higher thermal stability because of its complex heterogeneous
structure.[53]